1. Technical Field
The present invention relates to an autofocus system, particularly, to an autofocus system which does not cause a large difference in focus even during high-speed zooming.
2. Related Art
In television cameras for broadcast or business, there are cameras equipped with an autofocus (AF) system for automatically bring a desired subject into focus. In such camera systems, generally an AF function based on a contrast method is employed, and focusing is performed in a way that the focus of a photographing optical system is controlled so that a captured video has the maximum (largest) contrast by detecting a height of the contrast thereof.
Patent Document 1 (JP-A-2007-232793 corresponding to US-A-2007/0201851) and Patent Document 2 (JP-A-2003-075717) disclose that autofocus methods for controlling the focus based on the lens-to-subject distance to a target subject by detecting the lens-to-subject distance based on the size of the target subject to be brought into focus.
Furthermore, for example, Patent Document 3 (JP-A-2006-258944 corresponding to US-A-2006/0210260) discloses an autofocus system having a AF frame auto-tracking function by which an AF frame (an AF area) as an area brought into focus by the AF function automatically tracks a desired target subject (a tracking target subject) to be brought into focus.
However, in the general camera work of a cameraman, a photography target subject (the target subject to be brought into focus) is captured on a photographing screen by setting the zoom range to the wide side, and the zoom range is set to the telephoto side so that the target subject has an appropriate size on the photographing screen. At this time, the AF function based on the contrast method may be used. In this case, even in a state where it is determined that the target subject is appropriately brought into focus when the zoom range is set to the wide side, the range of the focal position, in which it is determined that the subject is visually in focus, is large. Hence, when the zoom range is rapidly changed to the telephoto side, it is difficult to follow the AF processing, and thus the target subject is out of focus by a large amount. Accordingly, a problem arises in that focusing is stopped in the above-mentioned state since the AF function is not available.
Further, similarly to Patent Documents 1 and 2, the lens-to-subject distance is detected based on the size of the target subject, and the AF method of controlling the focus based on the detected lens-to-subject distance is employed. With such a configuration, it may be possible to handle the focus control based on the AF function in accordance with the rapid change in zoom range. However, in Patent Document 1, it is not considered that the focal position at which the target subject is precisely brought into focus is changed in accordance with the change in zoom range (the change in focal length).